1. Field of the Invention
The present invention relates to methods and apparatus using fluidized bed principles for separating mixtures of solid articles of different densities, and more particularly to such methods and apparatus as are applicable to the grading of agricultural products or the separation of agricultural products from associated waste materials.
2. Background Art
The use of density variation as a means of separating mixtures of articles is widespread. In agriculture, the separation and sorting of produce on this basis is accomplished using both wet and dry methods.
Wet methods use a liquid as a medium with which to separate denser articles, which sink in the given liquid, from the lighter ones that will float thereupon. Because of the use of fluids, however, these techniques have disadvantages which limit their application with agriculture products. Some liquids employed are expensive or present fire and social hazards when used in large quantities. In addition, some agriculture commodities require prewetting in order to remove air bubbles and thereby permit their effective sorting in fluids. Other products are not susceptible to processing in any liquid because the absorption of liquid adversely effects the properties of the product. Finally, the liquids involved frequently become contaminated with foreign materials during the sorting process, effecting their density and requiring periodic changing or filtering.
Dry methods of sorting or cleaning of agriculture products are not afflicted by the above-described disadvantages. Some dry methods of sorting employ a form of pneumatic separation based on a combination of differing densities and differing aerodynamic properties associated with the components to be sorted. In such separation schemes, a gas, such as air, is forced upwardly through a moving bed of the mixture to be separated. This gas flow through the interstices of the particles of the mixture tends to disengage the particles from each other, permitting the gas flow to support at least some of the weight thereof. As a result, the bed of the mixture resembles a liquid of high viscosity, and the particles of the mixture are freed to a degree to migrate within the bed under the influence of physical forces that might tend to induce separation among the constituent components. In this respect, such methods employ fluidized bed principles.
The separation that occurs when a mixture to be separated is itself fluidized is not one that results exclusively due to differing density among the components of the mixture. Instead, the aerodynamic properties of the particles of the mixture also have a substantial impact upon the rate and quality of the separation that results. The upward flow of gas through the mixture will tend to draw with it the less compact particles of the mixture, regardless of their density.
Typically, the fluidization of such a mixture is effected as it passes down an inclined trough. At the discharge end of the trough the mixture of the materials has become somewhat stratified according to the combined density and aerodynamic property of the component particles. Nevertheless, such devices have several inherent drawbacks which render them less than optimally desirable in relation to the broad range of circumstances in which agriculture separators of the dry variety are nevertheless desirable.
First, separators which pneumatically fluidize the actual mixture to be separated have limited separation effectiveness. While the upper and lower layers of the stratified mixture discharged from the end of the separator trough may be relatively pure, the layers intermediate thereto continue to comprise a mixture of particles of both densities. This problem is ameliorated to some degree by horizontally narrowing the separation between the vertical walls of the trough in the vicinity of its discharge end. This has the effect of increasing the depth of the flow at that point, affording more vertical distance between the separated top and bottom layers of the mixture. Still, at some point between these two extreme layers, the two materials of differing densities remain substantially intermixed in an interfaced layer. This fact precludes the achievement of optimal separation effectiveness.
A second, more profound drawback of separation methods in which the mixture to be separated is itself pneumatically fluidized arises from the fact that fluidization of the mixture is not possible if the particles of the mixture have diameters greater than approximately three or four millimeters. Thus, such methods are effective only in separating small products, such as grain cereal. They cannot be used to separate or sort large produce.
Toward that end, resort has been made to the use of fluidized beds which are constituted of a material other than the mixture to be separated. For the purpose of separating mixtures of larger solid bodies of differing densities, a fluidized bed created from such a fluidization medium behaves in a manner analogous to a liquid, but without wetting the articles of the mixture it is used to separate. Pieces of solid material less dense than the apparent density of the fluidized bed will float on the surface thereof. These will hereinafter be referred to as the "float fraction" of such a mixture. Pieces of solid material which are more dense than the apparent density of the fluidized bed will on the other hand sink to the bottom of the bed. These will hereinafter be referred to as the "sink fraction" of such a mixture.
For such separation to occur, the apparent density of the fluidized bed must be intermediate the densities of the float and sink fractions of the mixture. Additionally, the particle size of the fluidization medium must be smaller by several orders or magnitude and the size of the bodies contained in the mixture.
The use of a fluidization medium other than the mixture to be separated advantageously reduces the influence on the process of other separation factors, such as aerodynamic characteristics, and reduces the process to one in which separation is accomplished substantially on the basis of differing density only. In addition, the presence of a layer of fluidization medium intermediate the float fraction of the mixture on top of the fluidized bed and the sink fraction of the mixture at the bottom thereof permits a clean separation of the float and sink fractions. This is accomplished by separating the upper portion of the fluidized bed with the float fraction entrained therein from the lower portion thereof having the sink fraction entrained therein. Thereafter the two components are cleaned independently to remove any fluidization medium, and close to one hundred percent separation effectiveness between the float and sink fractions of the mixture can be achieved.
A dry method separator of this type, which is particularly adapted to the sorting and cleaning of agriculture products, is disclosed in United States Patent Application No. 055,705 filed in the United States Patent and Trademark Office on May 29, 1987. In that device, a fluidization medium, such as sand, is used to create a moving fluidized bed of density intermediate the densities of the components of a mixture of agriculture products. This is accomplished in an inclined trough, the vertical sidewalls of which narrow toward the discharge end thereof in order to increase the depth of the fluidized bed as it leaves the trough. The mixture to be separated is added to the fluidized bed at an initial point in its flow near the upper end of the trough. Separation of the components of the mixture is effected while they are moving with the fluidized bed through the trough.
Less dense components of the mixture, constituting the float fraction thereof, rise to the surface of the fluidized bed and are displaced therewith along the length of the trough under the effect of the dynamic forces of the moving stream. Denser components, constituting the sink fraction of the mixture, settle to the bottom and are similarly advanced along the length of the trough. At the discharge end of the trough, the top portion of the fluidized bed entraining the float fraction of the mixture is discharged separately from an underflow of the fluidized bed which entrains therewith the sink fraction of the mixture. Appropriate means are then provided for cleaning the fluidization medium from the sink and float fractions of the mixture. The fluidization medium is then resupplied by a system of conveyor belts to the upper end of the trough. While the resulting apparatus and method result in a very high separation effectiveness, several problems remain.
First, the use of conveyors to resupply fluidization medium to the top end of the trough permits substantial unrecoverable losses of the fluidization medium during the recirculation cycle. Fluidization medium tends to fall from the edges of the conveyors involved and cannot be returned to the system. Thus, a gradual dissipation of fluidization medium occurs, requiring frequent replenishment from a reserve thereof. As the fluidization medium must necessarily exhibit a relatively consistent degree of cleanliness and uniform particle size, it can only on occasion be obtained on location. This renders its dissipation through operation of the separator a significant problem.
In addition, the use of conventional conveying belts to recirculate the fluidization medium, results in a separation apparatus of excessive size. Lengths in excess of 65 feet have been required to adequately recirculate fluidization medium. This has naturally effected the cost and mobility of separators constructed along such lines.
It has also been found that the fluidization medium utilized in such devices has a tendency to accumulate moisture, either from water contained in the mixture of articles to be separated, or from the ambient precipitation of moisture in the form of rain or dew onto the device when it is not in use. Moisture in the fluidization medium results in a fluidized bed of excessive viscosity, producing reduced separation effectiveness, as well as the clogging and rusting of the machinery involved. The latter can cause significant down time and require wholesale replacement of fluidization medium.
In addition, difficulty has been encountered in establishing an adequately deep fluidized bed to permit easy separation of sink and float fractions entrained therein. Often the process of deepening the fluidized bed requires a lengthy start up period. Depending on parameters of trough size, slope, or airflow, adequate depth has on occasion been found impossible to attain. In the case of slow depth attainment, operating time is increased undesirably. Where desired depth cannot be achieved, the efficiency of separation is sacrificed.
A second problem encountered in relation to the maintenance of a desired depth in the fluidized bed has been caused by the disruption in the depth of the bed created when the mixture to be separated is added thereto. In almost all circumstances, the constancy of the rate of supply of that mixture to the separator cannot be assured. The size of the particles in the mixture contributes to this difficulty. In addition, quantities of the mixture must be brought to the separator by truck in batches. The initiation and termination of the feeding of each batch to the separator dramatically alters the volume of the mixture added to the fluidized bed. This accordingly affects the depth of the bed, creating instabilities and in many cases reducing separation efficiency. Very often differing float or sink fraction densities in differing batches of mixtures alter the rate at which those sink and float fractions are cleared from the trough from which the fluidized bed is flowing. Any change in the rate of evacuation of either or both fractions of the mixture also affects the volume of material contained in the trough and, accordingly, the depth of the fluidized bed.
Fluidization in such devices is effected by forcing upwardly through the fluidization medium a gas, such as air. This is effected by using a perforated air distribution sheet as the floor of the trough. Air under pressure is supplied in a plenum on the underside of the air distribution sheet and percolates upwardly through the fluidization medium thereabove. The perforations size and density of the air distribution sheet must be carefully determined. The perforations must be small enough to prevent fluidization medium in the trough above the sheet from entering or clogging the air passageways through the sheet. Use of the separator, however, generates a great deal of dust. Thus, air forced through the air distribution sheet is frequently heavily laden with particles large enough to block the perforations therethrough. Such blockage reduces the volume of air that passes through the fluidization medium above the air distribution sheet, altering the resulting density of the fluidized bed created. When that density ceases to be intermediate the densities of the float and sink fraction of the mixture involved, the separator no longer operates effectively. Machine downtime is necessitated to permit the removal of such contaminants.